The molecular biology of the Harvey and Kirsten murine sarcoma virus p21 ras gene and its cellular homologs (c ras) has been studied. Reconstruction experiments between the viral Harvey ras and the rat c Harvey ras 2 gene have revealed a translational defect in the 3' half of the latter gene. A novel ras translational variant, p28 ras, is initiated from the rat c Harvey ras 2 promotor sequences. Studies of the expression of the c ras genes have revealed that both Harvey and Kirsten c ras genes are expressed by means of vastly differently-sized mRNA species, which apparently differ in the lengths of their 3' untranslated regions. Facilitated by the high levels of c Kirsten ras mRNA in a murine hematoporetic cell line, in vitro translation was performed on the two different c Kirsten ras mRNA species: Their p21 translational products were undistinguishable.